A year after Yahoo struck a deal with Mozilla to make its search engine the default in Firefox in the U.S., Yahoo's share of search usage share has fallen to levels lower than before the partnership was inked, according to one Web measurement.

In the last five months, Yahoo's U.S. search share as tracked by Irish metrics vendor StatCounter has has been under the 8.9% recorded in November 2014, the month when Yahoo and Mozilla announced the deal. For October, Yahoo's usage share was 7.6%, a new low. By StatCounter's tally, Yahoo's top mark in the past 12 months was the 10.2% in January 2015.

Other measurements also showed a decline in Yahoo's U.S. search share, albeit with much smaller losses. comScore, for example, put Yahoo's share at 12.5% for October, up from the 10.2% in November 2014, but down from the January 2015 peak of 13%.

Like StatCounter, comScore's data showed that Yahoo's U.S. search share slipped the last two months after holding steady from May through July.

Mozilla changed the default search from Google to Yahoo when it released Firefox 34 on Dec. 1, 2014. The Mozilla-Yahoo deal was a result of the former not renewing its long-standing partnership with Google, which in 2013 generated approximately $275 million in revenue for Firefox's developer.

Google continues to place a plea for U.S. Firefox users to return to the fold when they execute a search on google.com. "Get to Google faster. Switch your default search engine to Google," the on-screen message stated after a Firefox search.

It's not surprising that Yahoo's search strength in the U.S. has recently declined: Firefox's user share -- a proxy for the number of desktop and notebook personal computers that use the browser as tracked by analytics company Net Applications -- has slipped in the last two months.

Firefox's global 11.3% user share of October was down from August's 11.7%, a four-tenths of a percentage point decline that represented a 3.4% reduction. In the last 12 months, Firefox has lost 2.6 percentage points in user share, a slide of 19%.

The Yahoo-Mozilla contract will expire near the end of 2018.

Data: StatCounter

Thanks to the Firefox deal, Yahoo's U.S. search share climbed at the end of last year and the beginning of this year. But the troubled company has recently seen its domestic usage share slump as Bing replaced it in the second spot, far behind the No. 1 Google.

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